It has heretofore been known that vinyl compounds, such as styrene, acrylic acid, methacrylic acid, esters of acrylic acid, esters of methacrylic acid, and acrylonitrile, are easily polymerized by light or heat. In processes for producing vinyl compounds, various types of distillation are conducted for separation and recovery, concentration, or purification of the desired vinyl compounds. However, because vinyl compounds form polymeric materials by polymerization by light or heat as described above, various problems arise in the distillation process and cause various undesirable phenomena, such as that a stable continuous operation for a long time is made impossible.
Therefore, distillation operation has heretofore been conducted in the presence of a polymerization inhibitor in order to avoid these problems caused by polymerization in the distillation process. As the polymerization inhibitor, for example, copper dibutyldithiocarbamate, hydroquinone, methoquinone (p-methoxyphenol), p-t-butylcatechol, t-butylhydroquinone, or phenothiazine is used. However, these polymerization inhibitors exhibit small or very limited effect to inhibit polymerization in parts formed by condensation of the gaseous phase although a relatively desirable effect is exhibited in parts of the liquid phase. Therefore, the problem that, in a distillation tower, formation of polymerized materials cannot be suppressed in parts where the fluid containing a polymerization inhibitor is not sufficiently supplied has persisted. There has also been another problem that copper dibutyldithiocarbamate described above cannot readily be used in an industrial plant because this compound corrodes apparatus, such as those made of SUS 316.
On the other hand, a process for inhibiting polymerization of acrylic acid by using a cobalt, nickel, or manganese complex of N-nitrosophenylhydroxylamine has been proposed (the specification of the U.S. Pat. No. 4,638,079. However, this process has a drawback in that the effect of inhibiting polymerization is not sufficiently exhibited in a system containing both of the gaseous phase and the liquid phase, such as the distillation process of acrylic acid.